1. Field of the Invention
The present invention relates to a zoom optical system, and more particularly to a large aperture zoom optical system and to an image pickup apparatus incorporated with the large aperture zoom optical system.
2. Description of the Background Art
A variety of types of large aperture zoom optical systems have been developed and proposed. Typically, there are known zoom optical systems (zoom lens units) as disclosed in patent literatures 1 to 3.
The zoom lens unit disclosed in patent literature 1 is provided with a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a fourth lens group having a positive refractive power successively from the object side. The fourth lens, group has a front group having a positive refractive power, and a rear group having a positive refractive power. At the time of zooming from a wide angle end to a telephoto end, the interval between the first lens group and the second lens group increases, and the interval between the second lens group and the third lens group and the interval between the third lens group and the fourth lens group non-linearly change. The zoom lens unit is provided with a displacement member for preventing vibrations by moving the front group of the fourth lens group in a direction substantially orthogonal to the optical axis.
The zoom lens unit disclosed in patent literature 2 is provided with four lens groups i.e. a first group having a positive refractive power, a second group having a negative refractive power for zooming, a third group having a positive refractive power for correcting an image plane which changes as the magnification is varied, and a fixed fourth group having a positive refractive power successively from the object side. The first group has a first sub group having a positive refractive power, and a second sub group which has a negative refractive power and is moved along the optical axis for focusing. The second sub group has a positive lens element 12P convex toward the image side and a biconcave negative lens element 12N.
The large aperture ratio inner focus type telephoto zoom lens unit disclosed in patent literature 3 is provided with a first lens group G1 having a positive refractive power, a second lens group G2 having a negative refractive power, a third lens group G3 having a positive refractive power, and a fourth lens group G4 having a positive refractive power successively from the object side. The second lens group G2 and the third lens group G3 are moved along the optical axis for zooming. The first lens group G1 is composed of a front group G1F having a positive refractive power, and a rear group G1R having a positive refractive power stronger than that of the front group G1F successively from the object side. The rear group G1R in the first lens group G1 is composed of a meniscus negative lens element convex toward the object side, and a positive lens component successively from the object side. The rear group G1R in the first lens group G1 is moved along the optical axis for focusing. The zoom lens unit disclosed in patent literature 3 meets the following conditional expression:0.005<f1R/(f1F*D1)<0.055
where
f1F: a focal length of the front group G1F in the first lens group G1;
f1R: a focal length of the rear group G1R in the first lens group G1; and
D1: a distance along the optical axis between a lens surface of the front group G1F, closest to the image side, and a lens surface of the rear group G1R, closest to the object side when the lens unit is in the infinity in-focus state.
The aforementioned typical zoom optical systems (zoom lens units) have positive-negative-positive-positive four lens group arrangement, or a five lens group arrangement if the divided groups are counted individually. At the time of zooming, the first group and a group closest to the image side are fixed, and at the time of focusing, the first group and an image-side subgroup (e.g. the second sub group) obtained by dividing the first group into two are moved in the optical axis direction for focusing.
Recently, digital cameras have been generally used as a camera (image pickup apparatus), in place of so-called silver-halide cameras (film cameras). Further, in such digital cameras, moving image photographing is enabled, in addition to still image photographing by auto-focusing. In the moving image photographing, normally, sounds such as voices are simultaneously recorded, as well as images. In view of this, it is required to suppress the sounds such as operation sounds caused by a camera or an imaging lens (imaging optical system) as much as possible. Generally, sounds caused by an imaging lens are operation sounds caused by an actuator, which is used for focusing or camera shake correction. It is necessary to suppress these operation sounds of the actuator as much as possible. Therefore, it is essentially important to reduce the load of an actuator for suppressing the sounds by reducing the operation sounds of the actuator. Thus, there is a demand for reducing the weight of an optical system which is required to move at the time of focusing.
The zoom lens units disclosed in patent literatures 1 to 3 are constructed in such a manner that an optical system which is moved at the time of focusing is composed of plural lens elements. Therefore, the zoom lens units disclosed in patent literatures 1 to 3 are technically insufficient in the above aspect.
In addition to the above, generally, the total length of a zoom optical system is relatively long, which is not suitable for carrying. Thus, there is also a demand for miniaturization.    Patent Literature 1: JP Hei 02-244110A    Patent Literature 2: JP Hei 07-092431A    Patent Literature 3: JP 2000-019398A